The pH of a solution is a measure of the molar concentration of hydrogen `(H^+)` , or hydronium `(H_3O^+)` ions of the solution. If a solution has a hydronium ion concentration of `[H^+]` , the pH of the solution is equal to `-1*log_10[H^+]` .
The pH of pure water at STP is equal to 7. Acids have a pH less 7.
When phosphoric acid `H_3PO_4` is diluted with water it dissociates as follows:
1) `H_3PO_4 -> H^+ + H_2PO_4^-`
2) `H_2PO_4^(-) -> H^+ + HPO_4^(2-)`
3) `HPO_4^(2-) -> H^+ + PO_4^(3-)`
The acid dissociation constants of (1), (2), and (3) are equal to 7.1*10^-3, 6.3*10^-8 and 4.5*10^-13 resp.
To determine the pH of a 0.1 M solution of phosphoric acid, the concentration of H^+ in the solution has to be determined. The first deprotonation step leads to the formation of 0.1 M of hydrogen ions.
The second and third deprotonation steps can be ignored for acid concentrations greater 0.001 M as the [H^+] due to these is negligible.
Now Ka1 =...
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